The low pH of the stomach provides the host with a protective barrier to infection and must be overcome to allow oral vaccines to be effective. Salmonella and other enteric bacteria have developed strategies to surmount this harsh environment. In Salmonella, low pH also serves as a signal to the incoming bacterium resulting in the up regulation of a number of genes that facilitate its interaction with the host. Recombinant attenuated Salmonella vaccines are an effective and inexpensive way to elicit strong mucosal and humoral immune responses. However, many Salmonella Typhi vaccines are more sensitive to acid than the wild-type strain and have poor efficacy. Current strategies rely on ways to neutralize or by-pass stomach acid, which may result in reduction of the vaccine's ability to stimulate a strong immune response. We will investigate ways to enhance the acid resistance of vaccine strains to eliminate the need for neutralizing or bypassing the low pH environment of the stomach. We will modify an existing Salmonella acid resistance system to up regulate its expression and activity and import two acid resistance systems from other bacteria. We will evaluate these systems in several different attenuated, acid sensitive Salmonella strains for the ability to confer increased acid resistance in vitro. We will develop a 'low pH' mouse system in which we will model the pH of the human stomach providing us with an in vivo system to test the acid-resistant strains. We will evaluate the effects of acid-resistance on the immunogenicity of orally administered S. Typhimurium strains with genotypes similar to some current recombinant and non-recombinant S. Typhi vaccine candidates. Increasing acid resistance represents a means to reduce the efficacious dose and enhance the immunogenicity of live S. Typhi vaccines. PUBLIC HEALTH RELEVANCE: Effective live recombinant attenuated Salmonella vaccines expressing heterologous antigen genes hold the promise of providing low cost, orally administered, life-long protection against a variety of diseases. However, while effective in animal models, the results from human trials have been disappointing due to poor immune responses in vaccines. The goal of this project is to enhance the immunogenicity of live, oral Salmonella vaccines by increasing resistance to the low pH environment of the human stomach.